Abstract

This paper describes frequency modulation of a $cw$-dye laser to generate a frequency-discriminant dispersive signal by phase-sensitive detection of the optogalvanic signal in a lanthanum hollow cathode lamp. The frequencies of the six major hyperfine components in $5{{d}^{2}}6s\,{^{4}F}_{9/2}\ (4121.572\,\,{{\rm cm}^{-1}})\mathop{-\!\!-\!\!-\!-\!\!\rightarrow}\limits^{579.13\,{\rm nm}}5{{d}^{2}}6p\,{^{4}{{F}}_{9/2}}\,(21384.0\,{{\rm cm}^{-1}})$ transition in ${^{139}{\rm La}}$ I were measured with an accuracy of 50 MHz, using a calibrated wavemeter after locking the $cw$-dye laser frequency to the dispersive error signal of the constituent hyperfine component. The measured hyperfine separations are found to be consistent with separations measured using the standard amplitude modulation technique and also with earlier reported results. The relative ease of the optical setup for locking of the $cw$-dye laser to optogalvanic spectra can be adapted to atomic physics applications for stabilization of laser frequency in the visible region.

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